To resolve these problems numerous processes for treatment of the spent sand have been proposed for the purpose of recovering it, or at least of reducing the polluting charge, among which the most common are the wet process and the roasting or calcining process.
In the first case, the spent sand the granules of which are covered in particles of carbon and organic material which constitutes the residue of the chemical binding materials utilised to prepare the moulds for casting, are dispersed in an aqueous medium where the friction between the granules due to the irregular motion of these in the aqueous medium causes mechanical removal of the polluting substances (in particular carbon) which then stratify, by the effect of the different specific gravities, permitting the clear separation of the two solid phases (cleaned sand and removed residues); this process is very simple from the equipment point of view, but has the disadvantage of leaving the sand wet so that it is necessary to dry it, with consequent increase in costs; moreover, this process is particularly suitable for the so-called "black earth" where the majority of the residue is carbon, but is less effective in the case of organic pollutants.
In the second case, see e.g. U.S. Pat. No. 2,553,318, roasting to calcination of the spent sand is obtained by treating it with a current of hot gases in such a way as to cause elimination of the organic and carbon residues in gaseous form by direct or subsequent pyrolysis and/or combustion. The possibility of recovering the heat of combustion of these substances for pre-heating the combustion air makes it possible to contain the operating costs. On the other hand the costs of the installation are rather high in that the process is typically performed in rotating furnaces or, more frequently, by means of fluidised bed installations. Therefore this type of process can only be economically performed using large scale installations, which cannot be managed by medium and small foundries (which are those which generate a good part of the spent sand) at least without recourse to cooperatives or other similar community initiatives. However, even in this case the method of calcination is not free from disadvantages: first of all it is constrained to work in batches in that it is not possible to mix sand having different provenances to effect the treatment because this would involve the risk, for example, of mixing basic sand with acid sand, with the obvious consequences. Moreover, if there are present in the sand residues of low melting point metals (Cu, Zn, Sn, Pb) these can melt during the treatment, percolating through the fluidised bed and obstructing the holes through which the gas flow sustaining the calcination is delivered. As a direct consequence, practically all spent sand coming from non-ferrous metal workings, which are the most polluted, cannot be treated with this method.